Low noise high thermal conductivity mixed signal package

ABSTRACT

An improved microelectronic assembly ( 100 ) and packaging method includes a device package for housing a semiconductor die or chip, ( 105 ), an array of passive electronic components ( 305 - 355 ) operating in cooperation with the flip chip semiconductor die ( 105 ) and housed inside the device package to decouple noise from input signals, and a heat spreader ( 195 ) disposed between a top surface of the semiconductor die ( 105 ) and a package cover ( 185 ). The semiconductor die ( 105 ) is configured as a flip chip die and the device package includes a package substrate ( 110 ) configured as a ball grid array. The improved microelectronic device ( 100 ) reduces parasitic inductance in electrical interconnections between the semiconductor die and an electrical system substrate ( 115 ) and reduces signal noise in mixed signal high frequency analog to digital converters operating at clock rates above 1 GHz.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/508,106 entitled LOW NOISE HIGH THERMAL CONDUCTIVITY MIXEDSIGNAL PACKAGE filed on Jul. 23, 2009, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved microelectronic assemblyand packaging method. In particular, the improved microelectronicassembly includes a device package for housing a semiconductor die orchip, an array of passive electronic components operating in cooperationwith the semiconductor die and housed inside the device package, and adiamond heat spreader in contact with the semiconductor die to improvethe flow of heat.

2. Description of the Related Art

Electronics systems assembled on a circuit substrate, e.g. a printedcircuit board, (PCB), or the like, generally include one or moremicroelectronic assemblies and a plurality of passive electroniccomponents surface mounted onto the PCB. The microelectronic assembliesand passive components are electrically interconnected by “PCBinterconnections” comprising a series of conductive pathways such asconductive planes or conductive runs or buses on various layers of thePCB and interconnected by via holes, and the like. Typically, eachmicroelectronic assembly includes one or more integrated circuits,(IC's), formed on a semiconductor die or chip and a device packagecomprising a housing or enclosure surrounding the chip to mechanicallysupport the chip and protect the chip from damaging mechanical andelectrical shock, contaminates and moisture. In addition, the devicepackage may block damaging electromagnetic radiation transmission andfacilitate thermal energy dissipation. Moreover, the device packageincludes “package interconnections” comprising a series of conductiveelements forming conductive pathways that extend from the semiconductorchip to the PCB for electrically interconnecting the IC's with theelectrical system.

In many electrical systems, passive components are incorporated into thesystem to correct unavoidable shortcomings of system performance. Inparticular, passive components such as resistors, capacitors andinductors filter signal noise, damp circuit resonances and stabilizesignal frequencies. An important example of such a correction is adecoupling circuit. Decoupling circuits are typically used inassociation with high frequency digital logic and mixed signal circuitssuch as computer mother boards, digital cameras, and other digitalimaging systems.

Typically, the decoupling circuit comprises one or more capacitorselectrically interconnected between a power distribution system or powersupply and an IC, such as an analog to digital converter (ADC), housedinside a microelectronic assembly. Digital circuits have high powerdemands synchronized with clock pulses and a low power demand betweenclock transitions. This occurs because clock pulses initiate millions oflogic steps all drawing power simultaneously. The decoupling capacitorsstore charge between clock pulses and deliver the charge when the clockpulse occurs. This decouples power supply switching noise and othertransients from IC signals and maintains a substantially uniform inputsupply voltage.

As input signal and IC clock frequencies increase, the number ofdecoupling capacitors needed to decouple power supply transients alsoincreases to the point that high frequency mixed signal IC's may requireas many as 50 decoupling capacitors taking up valuable space on the PCB.Generally, there is a need in the industry to reduce the number ofpassive components on PCB's and especially on space limited PCB's usedin small devices such as a hand held device, e.g. cell phones, and otherconsumer electronic products.

Another problem associated with the use of capacitors on PCB's is thatcapacitors interact with the parasitic or self inductance and resistanceinherent in the conductive pathways electrically interconnecting thecapacitors with the IC's. The parasitic inductance and resistance whencombined with the capacitance of the decoupling capacitors act like anR-L-C network having resonant frequencies and harmonics capable ofinjecting additional noise into IC input signals and possibly capable ofdamaging the IC and/or adversely affect circuit performance. Inparticular, “PCB interconnections” and “package interconnections” have“self inductance” or “parasitic inductance” that interacts withdecoupling capacitors to form an R-L-C network. Moreover, the magnitudeof the reactance (i.e., the impedance due to the parasitic inductance)is proportional to the input signal frequency with the reactanceincreasing with increasing input signal frequency. In particular, it isknown that special decoupling circuits for IC's operating with inputsignals having a frequency above 50 MHz require multiple fast actingdecoupling capacitors operating in parallel in order to keep up withcharge demands. Recently, as input signal frequencies begin to exceed 1GHz, the interaction of decoupling capacitors with the parasiticinductance of PCB and package interconnections have become problematicas the R-L-C networks formed by decoupling capacitors on a PCB andpackage interconnections generate unacceptable signal noise.

Conventional packages are unacceptable for packaging new high speed,high dynamic range (i.e., high signal to noise ratio), high powerdissipation, mixed signal die because there are no conventional packageswhich simultaneously address all 3 of the following requirements: 1)maintain sufficiently low junction temperatures for the high powerdensities and high overall power dissipation of the new die, 2) containsufficiently low parasitics in the interconnects within the IC packageto achieve a low noise environment and high dynamic range operation inthe presence of high speed switching and mixed analog/digital circuitry;and 3) contain adequately short path lengths between the die andcritical passive components (such as decoupling capacitors) to minimizeparasitics for proper operation of the high speed, high dynamic rangecircuit.

SUMMARY OF THE INVENTION

The present invention overcomes the problems cited in the prior art byproviding a microelectronic assembly that reduces the parasiticinductance of “package interconnections” and eliminates the parasiticinductance of “PCB interconnections” between decoupling capacitors andthe microelectronic assembly by housing the decoupling capacitors insidethe device package.

The die thermal management problem is addressed by utilizing a high(thermal) conductivity heat spreader and removing the heat through thepackage lid. This thermal solution is compatible with the low parasitic,short interconnect path length, flip chip die approach, thus solvingboth the thermal and electrical challenges in one package. Themicroelectronic assembly solves the problem by combining high densitypackage interconnects, in-package passive components (such as decouplingcapacitors), flip chip semiconductor die attach, and a high thermalconductivity heat spreader coupled to a high thermal conductivity lid ina ball grid array package.

In particular, the microelectronic assembly includes a semiconductor diehaving one or more integrated circuits formed thereon for processinginput signals, capacitors for decoupling noise on supply voltage busesand tunable inductors to allow tuning of flip chip componentperformance. The semiconductor die is packaged in a device package thatincludes at its base, a package substrate. The package substratemechanically supports the semiconductor die and electrically interfaceswith the integrated circuits formed on the semiconductor die as well aselectrically interfacing with an electrical system substrate to receiveinput signals from a larger electrical system and to deliver the inputand output signals to and from the IC. In a preferred embodiment, thesemiconductor die comprises a flip chip die having an array of studbumps extending from its bottom surface and configured as IC I/O ports.Moreover in the preferred embodiment, the package substrate comprises aball grid array package having an array of solder balls extending fromits bottom surface and configured as electrical interconnecting pointsfor electrically interconnecting the device package with an electricalsystem substrate such as a printed circuit board (PCB).

The device package includes a stiffening member such as a square ringformed by a continuous side wall comprising a metal e.g. stainlesssteel, copper, aluminum, or other material having a similar stiffness.The continuous side wall has an outer perimeter forming outside edges ofthe device package and an inner perimeter enclosing a hollow cavitysurrounding the semiconductor die. The continuous side wall has a bottomsurface fixedly attached to a top surface of the package substrate by anadhesive layer such that the continuous side wall stiffens the packagesubstrate in order to prevent the semiconductor die from bending ortwisting which could lead to die and/or stud bump cracking duringthermal expansion or handling. The continuous side wall thickness orheight above the package substrate exceeds a height of the semiconductordie.

The device package includes a cover fixedly attached to a top surface ofthe continuous side wall, e.g. by a layer of adhesive. The cover sealsthe hollow cavity thereby protecting the semiconductor die fromcontamination and moisture. In addition, the device package includes aheat spreader disposed between a top surface of the semiconductor dieand a bottom surface of the cover and in thermal contact with both. Theheat spreader is fabricated from a high thermal conductivity material,such as diamond which generally has a thermal conductivity coefficientof over 900 W/m° K at 20° C. The heat spreader has a cross-sectionalarea that is larger than the cross-sectional area of the semiconductordie but that still fits inside the hollow cavity. It should be notedthat in some applications, the heat spreader can provide effectivethermal management even if it does not have a surface area larger thanthe surface area of the top of the die (e.g., in certain applications,it may be advantageous to have the heat spreader surface area equal tothe top surface area. The heat spreader functions to conduct thermalenergy away from hot spots in the semiconductor die top surface, and toconduct thermal energy toward the cover. Preferably the cover is formedfrom copper which generally has a thermal conductivity coefficient ofover 400 W/m° K at 20° C. and functions to dissipate thermal energy tosurrounding air and to conduct thermal energy from the heat spreader tothe continuous side wall. The side wall serves to dissipate thermalenergy to the surrounding environment.

The device package includes one or more passive electrical componentshoused inside the hollow cavity. The passive electrical components mayinclude capacitors and or inductors electrically interconnected to thepackage substrate or to the semiconductor die by ribbon bondedconnections. Other electrical interconnections such as wire bonding orsolder bumps may be utilized depending on the selected passivecomponents. Certain passive electrical components function to preventnoise from contaminating the die input/output signals and prevent noisefrom interfering with proper IC operation. More specifically a pluralityof decoupling capacitors may be electrically interconnected with powerplanes and ground planes of the package substrate to decouple signaltransients from power supply inputs to the die. In addition, otherpassive components, such as high Q inductors may also be interconnectedwith the package substrate in order to realize other circuit functions,such as adjustable resonant filters.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention will best be understood from adetailed description of the invention and a preferred embodiment thereofselected for the purposes of illustration and shown in the accompanyingdrawings in which:

FIG. 1 illustrates a top view of selected elements of a microelectronicassembly according to the present invention.

FIG. 2 illustrates a side section view taken along line 2-2 through themicroelectronic assembly of FIG. 1 according to the present invention.

FIG. 3 illustrates an enlarged side section view of FIG. 2 through aportion of a microelectronic assembly according to the presentinvention.

FIG. 4 illustrates an enlarged side section view of FIG. 3 taken througha planar capacitor according to one aspect of the present invention.

FIG. 5 illustrates a top view of a dual tunable planar inductoraccording to one aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, a microelectronic assembly 100 is shown intop view with its cover 185 partially cut away and heat spreader 195removed. The microelectronic assembly 100 includes a flip chipsemiconductor die 105 packaged in a device package 210 that preferablyconforms to Joint Electron Device Engineering Council, (JEDEC) standardmicroelectronic device package dimensions of 25×25 mm square byapproximately 2.9 mm thick, but other package dimensions are usable. Themicroelectronic assembly 100 includes an array of passive decouplingcapacitors 305, 310, 315, 320, 325, 330 and 335 housed inside the devicepackage 210 and electrically interconnected with the semiconductor die105 using low inductance electrical interconnections.

The microelectronic assembly 100 may further include other components,such as an array of tunable, low profile, high frequency or high Qresonator inductors 340 and 350, electrically interconnected with thesemiconductor die 105 as part of resonator filter circuits. In addition,the microelectronic assembly 100 is constructed to eliminate wire bondedelectrical interconnections between the semiconductor die 105 and apackage substrate 110 in order to reduce parasitic inductance. Moreover,the microelectronic assembly 100 is constructed with a diamond heatspreader 195 (FIG. 2) disposed between a top surface of the flip chipsemiconductor die 105 and a cover 185 to improve thermal energymanagement within the device package.

The microelectronic assembly 100 is particularly suitable when thesemiconductor device 105 is configured as a mixed signal analog todigital converter (ADC) operating at high frequency. In particular, apreferred semiconductor die 105 includes integrated circuits (IC's)formed thereon for receiving a plurality of alternating current (AC) andvariable analog voltage input signals with average input signalvariations having frequencies of 1.3 GHz, for converting the inputsignals to digital values and for outputting equivalent digital signalsat an average data rate of 2 Gbits/sec and with a target effectivenumber of bits (ENOB) of 16 and with a target signal to noise ratio(SNR) of 98 dB in a 10 MHz bandwidth. In particular, the preferred ADCincludes a clock operating at 3.8 GHz. In addition, the microelectronicassembly 100 is configured to dissipate 16 watts of total power whenoperating at 60° C.

Referring now to FIGS. 2 and 3, a section view taken through section 2-2depicts the semiconductor die 105 supported on a package substrate 110,which is further supported on an electrical system substrate 115, (notshown in FIG. 1), such as a printed circuit board, (PCB), or the like.According to a preferred embodiment of the invention, the semiconductordie 105 is electrically interconnected with the package substrate 110and the package substrate 110 is electrically interconnected with theelectrical system substrate 115 without using wire bonded wires orribbons. In addition, a plurality of passive electrical components,capacitors (305, 310, 315, 320, 325, 330, 335) and dual planarinductors, (340, 350) are supported on the package substrate 110 andelectrically interconnected between the semiconductor die 105 and theelectrical system substrate 115 via electrical interconnections with thepackage substrate 110.

The semiconductor die or chip 105 comprises a thin wafer ofsemiconductor material cut with a square or rectangular cross-sectionalarea with opposing and substantially parallel top and bottom surfaces.In one example embodiment, the die dimensions are approximately 4.0mm×4.6 mm by 0.1 mm thick. As described above, the semiconductor die 105includes one or more integrated circuits (IC's) formed thereon and theintegrated circuits include one or more IC input/output ports or gatesformed as conductive areas or pads disposed on one or both of the topand bottom surfaces. In a preferred embodiment, the semiconductor die105 is configured as a flip chip having a plurality of conductive studbumps 120, e.g. comprising gold or silver, extending from thesemiconductor die bottom surface. The stud bumps 120 can be formed in atwo dimensional array of substantially uniformly spaced contact elementspositioned to mechanically support the semiconductor die 105 on thepackage support substrate 110 and to electrically interconnect the ICI/O ports to the passive components within the microelectronic assemblyand or to the package substrate 115. However, the pattern or array ofbumps may be either a uniform or nonuniform spaced array of contactelements.

Referring to FIG. 3, the package substrate 110 comprises a plurality ofthin conductive material layers 125 separated by a plurality of thindielectric or insulating material layers 130. The conductive anddielectric layers are disposed substantially horizontally and thepackage substrate 110 is formed with a square or rectangularcross-sectional area that exceeds the cross-sectional area of thesemiconductor die and preferably has dimensions of 25 mm square.Preferably the conductive material layers 125 comprise copper in anapproximate layer thickness of 6-25 μm and the dielectric layers 130comprise polytetrafluoroethylene, (PTFE) and or polyphenyl ether orpolyphenylene, (PPE), in an approximate layer thickness of 25-40 μm.

Generally each conductive layer 125 is laid out in a pattern ofconductive pads, conductive planes, runs, buses or other conductivepathways and each dielectric layer 130 separates two adjacent conductivelayers 125 to electrically isolate one conductive layer 125 fromanother. Moreover, the top and bottom surfaces of the package substrate110 comprise conductive layers 125. In addition to providing anelectrical interface between the semiconductor die 105 and theelectrical system substrate 115, the package substrate 110 providesenough mechanical stiffness to support the semiconductor die 105 and aswill be described below, a stiffening member 155 attaches to the packagesubstrate 110 to prevent the semiconductor die from bending or twistingduring handling or during thermal cycling and to support the packagecover required for protection of internal components and thermalmanagement. The stiffening member 155 is attached by bonding 132 to thepackage substrate 110.

The package substrate 110 top surface comprises a conductive layer 125formed with a plurality of isolated conductive pads, not shown,positioned to interface with each of the stud bumps 120. The packagesubstrate bottom surface comprises a conductive layer 125 formed with aplurality of solder balls 135, e.g. comprising tin, lead and mollifiedrosin, extending out therefrom. The solder balls 135 are formed in a twodimensional array of uniformly spaced apart elements, (e.g. 576 contactsin a 24×24 array spaced at a 1.0 mm pitch), positioned to mechanicallysupport the device package on the electrical system substrate 115 and toelectrically interconnect the package substrate 110 with the electricalsystem substrate 115. One skilled in the art will recognize that thestud bumps may be made from other materials and that the array size andpitch may vary for other applications.

Preferably each stud bump 120 is attached to the package substrate 110by conductive epoxy applied between the stud bump 120 and the topsurface of the package substrate 110. In addition, a conventional flipchip underfill layer 140, comprising a thermally conductive dielectricor other suitable material, is applied between the semiconductor die 105and package substrate 110 to improve thermal conductivity from thesemiconductor die 105 to the package substrate 110, to fixedly attachthe semiconductor die 105 to the package substrate 110, and to preventdie warping caused by forces generated by mismatches in the thermalexpansion characteristics and local temperature differences between thedie 105 and package substrate 110. Ideally, semiconductor die 105, thepackage substrate 110 and the underfill layer 140 are constructed frommaterials having substantially similar thermal expansioncharacteristics.

Referring to FIG. 2, the electrical system substrate 115 includes a topsurface 118 laid out in patterns of conductive pads, conductive planes,runs, buses, or other conductive pathways including a pattern ofconductive pads for interfacing with each of the solder balls 135. Inaddition, the electrical system substrate 115 includes electricalelements mounted thereon or connected thereto for forming one or moreelectrical systems for operating in cooperation with the microelectronicassembly 100. The solder balls 135 are attached to the electrical systemsubstrate 115 using any number of conventional attaching means such asby heating the solder balls 135 to a solder flow temperature to therebysolder the device package 210 to the electrical system substrate 115.Alternately, a conductive adhesive or other suitable attaching means areusable to attach and electrically interconnect the package substrate 110to an electrical system substrate 115 without deviating from the presentinvention.

Referring to FIG. 3, an enlarged section view of area 151 in FIG. 2shows a portion of the package substrate 110 and other elements in moredetail. As shown, each of the conductive layers 125 is electricallyinterconnected to other conductive layers 125 by blind via holes 145 andor through via holes 150. The via holes 145, 150 extend from oneconductive layer 125 to another by passing through one or moredielectric layers 130. The via holes 145, 150 are filled with, or coatedaround their inside diameters with a conductive material, e.g. copper,to provide a conductive pathway that extends along their longitudinallength. Accordingly, each via hole may electrically interconnect two ormore conductive layers 125.

The package substrate 110 is a high density interconnect (HDI) tominimize package parasitics by minimizing inductive loop lengths and bymaximizing decoupling capacitance between power and ground in thepackage interconnect substrate 110. The substrate 110 has short minimumdistances between adjacent vias 145, 150, has short minimum distancesbetween traces, and has short height between conductive layers 125.

The conductive layers 125 and via holes 145, 150 are laid out toelectrically interconnect each of the stud bumps 120 with one or moreconductive layers 125 as well as to electrically interconnect each ofthe solder balls 135 with one or more conductive layers 125 in a circuitpattern configured to exchange digital data signals, analog signals andpower signals between the IC's formed on the semiconductor die 105 andan electrical system formed on the electrical system substrate 115.Conductive layers and vias also interconnect with the passive parts inthe device package 210. In the preferred embodiment, each of thedifferent dc voltages and each of the uniquely-designated circuitgrounds has at least one large area metal plane on at least oneconductive layer 125 to maximize decoupling capacitance, minimizeinductance and minimize resistance within the package substrate. Morespecifically, the microelectronic assembly 100 receives power via sevendifferent power busses delivering seven different input power signals tothe semiconductor die 105. Each of the seven different input powersignals is delivered to at least one power plane in the packagesubstrate 100. Moreover, the seven different power supplies operate withfour different ground levels, and each of the four different groundlevels is connected to multiple ground planes in the package substrate100. However, other conductive layer layouts with different electricalrequirements are usable without deviating from the present invention.

Referring to FIGS. 1-3, the device package 210 further includes asupport member 155 functioning to stiffen the package substrate 110 andfurther functioning as a heat sink for storing and dissipating thermalenergy to surrounding air. The support member 155 comprises a squarering, e.g. stainless steel, titanium, or copper, formed by a continuousside wall, such as the four side walls 160, 165, 170, 175. Thecontinuous side wall forms an outside perimeter which matches an outsideperimeter of the package substrate 110 forming side edges of the devicepackage. The continuous side wall forms an inside perimeter forming sideedges of a hollow cavity 180 that surrounds the semiconductor die 105.The support member 155 is fixedly attached to the top surface of thepackage substrate 110 and functions to further stiffen the packagesubstrate 110. In addition, the support member 155 has a thickness inexcess of the thickness of the semiconductor die 105 such that a heightof the hollow cavity 180 exceeds the height of the semiconductor die 105and the diamond heat spreader 195.

In the preferred embodiment, the side walls 160, 165, 170, 175 form anouter perimeter with a square outside dimension of 25×25 mm and aninsider inner perimeter with a square inside dimension of 19×19 mm andan approximate thickness of 0.6 mm. This sizes the hollow cavity 180 toenclose the semiconductor die 105, the passive electronic components(305-350) and the heat spreader 195 therein. However, other devicepackage shapes and sizes are usable without deviating from the presentinvention.

Referring again to FIG. 2, the device package 210 includes a square orrectangular cover 185 sized to approximately match the square orrectangular dimensions of the package substrate 110 and the outerperimeter of the support member walls 160, 165, 170, 175. The cover 185is supported on a top surface of the support member 155 and attachedthereto by a thermally conductive adhesive layer 190. The cover 185functions to seal the hollow cavity 180 and also as a thermal conduitfor drawing thermal energy away from the heat spreader 195, conductingthe thermal energy to the stiffening member 155 and dissipating thethermal energy to the surrounding environment. The cover 185 ispreferably formed from copper, and other high conductivity materials maybe used for the cover.

The device package 210 further includes a heat spreader 195 sandwichedbetween the semiconductor die 105 and the cover 185. The heat spreader195 comprise a layer of material such as diamond having a high thermalconductivity that functions to rapidly conduct thermal energy away fromthe semiconductor die 105 and towards the cover 185. The heat spreader195 also functions to rapidly spread the thermal energy over its volume.For less demanding applications or where cost is a significant factor,other high thermal conductivity heat spreaders may be utilized such asmetal or ceramic-based including CuMo, Copper, BeO, and AIN. Preferably,the heat spreader 195 has a thermal conductivity of ≧900 W/m° K at 60°C., which is achievable when the heat spreader 195 comprises diamond.Preferably, the heat spreader 195 comprises a square or rectangularplate having a cross-sectional area with dimensions that meet or exceedthe cross-section dimensions of the semiconductor die 105 while stillfitting with the inside perimeter of the continuous side wall of thestiffening member 155. In particular, a preferred heat spreader 195 hasapproximate dimensions of 10.3×6.7 mm square by 0.5 mm thick. The heatspreader was larger than the die in the present embodiment. However, insome applications, the heat spreader may help provide thermal managementeven if it does not have a surface area larger than the surface area ofthe top of the IC (e.g., in certain applications, it may be advantageousto have the heat spreader surface area equal to the die top surfacearea).

A layer of thermal grease 200 is applied between the heat spreader 195and the cover 185. The thermal grease thickness is approximately 0.05 mmand the thermal grease provides low thermal resistance while decouplingstress due to mismatched coefficients of thermal expansion between theheat spreader 195 and cover 185. A layer of thermally conductive,compliant epoxy 205 is applied between the semiconductor die 105 and theheat spreader 195. The epoxy layer thickness is approximately 0.025 mmand the epoxy provides low thermal resistance while decoupling stressdue to mismatched coefficients of thermal expansion between the heatspreader 195 and the semiconductor die 105. As an example, adhesivemodel number 2600AT manufactured by Ablestik Inc. of Rancho DominguezCalif., USA provides the desired properties.

Referring to FIGS. 1 and 4, FIG. 4 shows a side section view of a planarcapacitor. The microelectronic assembly 100 of FIG. 1 includes an arrayof seven decoupling capacitors 305, 310, 315, 320, 325, 330, 335 and twodual passive inductors 340 and 350. In a preferred embodiment, eachdecoupling capacitor comprises a “planar capacitor” and each passiveinductor comprises a “planar inductor.” In particular, each planarcapacitor (FIG. 4) is formed on a flexible base substrate 230 such as aflexible polymer layer having a thickness of approximately 75 μM andhaving good thermal insulating properties. One such material used as aconventional flex-circuit substrate is sold by UBE Industries of Tokyo,Japan under the trade name UPILEX. Each planar capacitor comprises abottom conductive layer 235, e.g. a thin metal film, applied over theflexible base substrate 230, a dielectric layer 240, applied over thebottom conductive layer 235, and a top conductive layer 245, e.g. a thinmetal film, applied over the dielectric layer 240. In addition a passiveovercoating layer 250 is applied over the top conductive layer 245.

In a preferred embodiment, the bottom conductive layer 235 and topconductive layer 245 comprise copper having an approximate thickness of2 μm and the dielectric layer 240 comprises tantalum having anapproximate thickness of 5 μm with anodized surfaces forming a thinlayer of TaO₅ thereon. Each planar capacitor has a capacitance per unitarea of approximately 100-200 nF/cm² such that the capacitance ofindividual planar capacitors is determined by a cross sectional area ofthe capacitor. According to the invention, individual capacitors 305,310, 315, 320, 325, 330, 335 are formed with a cross sectional areacorresponding with a desired capacitance value in nF and may be sizedand shaped to fit together as a single layer of planar capacitorsattached to the top surface of the package substrate 110. As an example,the capacitors 305, 310, 315, 320, 325, 330, 335 have capacitance valuesranging from 12-112 nF. As further shown in FIG. 4, each planarcapacitor may be attached to the package substrate 110 by a layer ofadhesive 255 and electrically interconnected to the package substrate110 by two or more wire ribbon conductors 260. Preferably the wireribbon comprises gold wire ribbon having dimensions of 0.76×0.013 mm.Preferably each planar capacitor 305, 310, 315, 320, 325, 330, 335 isfabricated with less than 200 mΩ, parasitic resistance and less that 5nH parasitic inductance. Referring to FIG. 4, it is noted that theconductors 260 for electrically connecting the electrical component,here the capacitor 320, to the chip 105 through the package substrate110 are disposed under an overhanging portion of the thermallyconductive member 195. It is also noted that the conductors forelectrically connecting the inductors 340, 350 (FIG. 1) to the chip 105through the package substrate 110 also are disposed under an overhangingportion of the thermally conductive member 195. With such anarrangement, the capacitors and inductors can be positioned extremelyclose to the chip 105.

It is noted that the chip 105 is disposed in an inner region of thepackage while the passive elements (i.e., passive decoupling capacitors305, 310, 315, 320, 325, 330 and 335, and inductors 340, 350) aredisposed about an outer periphery of the chip 105. It is also noted thatthe conductors 235 and 245 provide the plates of the capacitors. Asshown in FIG. 1, these capacitor plates' inner edges 311 are positionedvery close to the chip 105 and have relatively short lengths; thesurface area of the plates then flare outwardly so that the plates'outer edges 313 are longer in length than the lengths of the inneredges. Thus, relatively large capacitance capacitors are formed yet areclose to the chip to enable the use of relatively short conductors 260(FIG. 4).

By minimizing the gap between the capacitor plate and the chip, arelatively low parasitic inductance results from the interconnectingconductor(s) between the capacitor plate and the chip thereby enablingthe chip to operate with higher clock rates. In order to shape theplates so that they can be positioned close to the chip and stillprovide a relatively large surface area to thereby increase capacitance,the width of the plates is increased as the plates extend from the gapregion near the chip outwardly from the chip towards the outerperipheral region of the package. A higher capacitance results inincreased charge that the capacitor can deliver and thus longer linevoltage maintenance without excessive droop.

Referring to FIG. 1, FIG. 4 and FIG. 5, the two dual planar inductors340 and 350 are constructed on the flexible base substrate 230 describedabove. In addition, a dual planar inductor comprises a bottom conductivelayer 235, e.g. 2 μm thick copper, applied over the flexible basesubstrate 230, an insulating layer 240, e.g. 4 μm thick Benzocyclobutene(BCB), applied over the bottom conductive layer 235, and a topconductive layer 245, e.g. 2 μm thick copper plated with nickel or gold.The planar inductors 340 and 350 may be electrically interconnected withthe package substrate 110 by the wire ribbon conductor 260 describedabove, and the planar inductors may be attached to the package substrate110 by an adhesive layer 255 shown in FIG. 4.

Referring to FIG. 5, dual planar inductor 350 is shown in top view; Dualplanar inductor 340 is similar to Item 350. The two inductors within adual planar inductor assembly are closely matched, and there is a commonground between the two inductors. Each dual planar inductor assembly,340 or 350, includes two input terminals 266 ₁, 266 ₂ one for eachinductor, and three ground terminals 265 ₁, 265 ₂, 265 ₃. The five inputand ground terminals are connected to the semiconductor die via toplayer metal on the package substrate 110. The inductors and connectedcircuitry within the semiconductor die form resonant circuits to tunethe mixed signal circuit to the correct operating frequency.

Preferably each inductor is designed for 3.0 nH and is adjustable toallow tuning of the resonant circuits. To achieve the tuning feature,the inductors in 340 and 350 include trim bars 270 that can be removedby laser trimming or otherwise to increase the inductance. Theinductance can be lowered by shorting across the inductor spiral.Ideally, each planar inductor is fabricated with a quality factorgreater than 50 and with a self resonance frequency of more than 8 GHz.

It will also be recognized by those skilled in the art that, while theinvention has been described above in terms of preferred embodiments, itis not limited thereto. Various features and aspects of the abovedescribed invention may be used individually or jointly. Further,although the invention has been described in the context of itsimplementation in a particular environment, and for particularapplications, e.g. as an analog to digital converter, those skilled inthe art will recognize that its usefulness is not limited thereto andthat the present invention can be beneficially utilized in any number ofenvironments and implementations where it is desirable to reduceinterconnection parasitics and decouple signal noise from an IC whilesimultaneously providing a high thermal conductivity solution.Accordingly, the claims set forth below should be construed in view ofthe full breadth and spirit of the invention as disclosed herein.

1. A method for packaging a microelectronic assembly for processing aninput signal comprising the steps of: supporting a flip chipsemiconductor die on a top surface of a ball grid array packagesubstrate; stiffening the ball grid array package substrate by fixedlyattaching a stiffening member to the top surface of the ball grid arraypackage substrate, wherein the stiffening member forms a hollow cavitysurrounding the semiconductor die; sealing the hollow cavity by fixedlyattaching a cover to a top surface of the stiffening member, wherein thecover comprises a thermally conductive material; spreading thermalenergy being dissipated by a top surface of the flip chip semiconductordie by disposing a heat spreader between the top surface of the flipchip semiconductor die and the cover wherein the heat spreader comprisesa thermally conductive material and further wherein the heat spreaderforms a thermal conduit extending from the semiconductor die to thecover; and, decoupling noise from the input power supply voltages byinstalling a passive electrical component inside the hollow cavity andby electrically interconnecting the passive electrical component to oneof the semiconductor die and the ball grid array package substrate. 2.The method of claim 1 wherein the stiffening member comprises stainlesssteel, the heat spreader comprises diamond and the cover comprisescopper.
 3. The method of claim 1 wherein the package substrate includespower planes associated with the power supply voltage inputs and one ormore ground planes and wherein the step of electrically interconnectingthe passive electrical component comprises connecting a planar capacitorin parallel between power plane and ground plane.
 4. The method of claim3 further comprising the step of connecting a planar inductor inparallel between the signal plane and the ground plane and wherein theplanar inductor has a self resonance frequency that is greater than twotimes an operating frequency of the semiconductor die.